The man appointed to lead Kenya's anti-corruption campaign has said early efforts to crack down on the problem are proving successful.

"The challenge is what major changes we can make in the next...18 months," presidential adviser John Githongo told BBC World Service radio.

Kenya's President Mwai Kibaki swept to victory in the December 2002 elections on a crusade against fraud and bribery and swiftly created the job of anti-corruption czar.

His government hopes it can restore international aid: the World Bank withdrew its support from Kenya in 2001 because the problem was so widespread.

Mr Githongo, who campaigned against corruption in his previous job as a newspaper columnist, said "gigantic patronage networks" had been disrupted by the initial crackdown.

'No witch-hunt'

The appointment of tribunals to investigate corruption in the civil service and police has sparked high profile resignations, including that of Chief Justice Bernard Chunga.

Mr Githongo said success would be harder to achieve "if we don't do some of the most difficult things now".

The disruption achieved so far was "temporary", he said.

Similar campaigns in other African countries have shown that networks of corrupt officials tend to regroup after about two years, he said.

Asked if he would pursue investigations against ex-President Daniel Arap Moi, he said: "It's important to reduce impunity ...at the same time it's important that the process of reducing impunity does not become perceived as a witch-hunt."

Mr Moi and his KANU party ruled Kenya for 39 years until they were voted out in the December 2002 election.

Kenyans are asked to pay a bribe in two out of every three encounters with public officials, according to the independent watchdog Transparency International.